Related subject matter is disclosed in the co-pending, commonly assigned U.S. patent application of E. Suhirxe2x80x941, Ser. No. 08/551,241, filed on Oct. 31, 1995, entitled xe2x80x9cData Carriers Having An Integrated Circuit Unitxe2x80x9d, in the co-pending, commonly-assigned U.S. patent application of Clifton-Flynn-Verdi 4-6-15, Ser. No. 08/558,579, filed on Oct. 31, 1995, entitled, xe2x80x9cSmart Card Having a Thin Diexe2x80x9d, and in U.S. Pat. No. 5,480,842 issued on Jan. 2, 1996 to Clifton, Flynn, and Verdi.
1. Field of the Invention
This invention relates generally to semiconductor devices, and more particularly to semiconductor die that are used in the manufacture of smart cards.
2. Background Art
Existing smart cards may fail when, due to applied mechanical stress, the semiconductor die of the smart card breaks. Mechanical stress is inherent in typical smart card operational environments, such as point-of-sale terminals, electronic cash machines, credit card reading devices, wallets, pockets, and purses. Semiconductor die strength is a significant factor in determining the overall durability and reliability of a smart card. Die thickness affects the ability of a semiconductor die to withstand flexure and applied mechanical force.
In the field of semiconductor fabrication, skilled artisans attempt to use the thickest semiconductor die that will fit within a smart card package. This approach is based upon an assumption that die strength is proportional to die thickness. Since existing smart card packages are approximately 0.030 inches thick, this dimension places a constraint on the maximum thickness of the semiconductor die which will fit within the package. To this end, note that it is not feasible to use semiconductor die that are about 0.030 inches thick. In addition to the die itself, the space within the smart card package is also occupied by lead terminations, structures that protect the die and/or the leads, labeling, magnetic striping, and discrete circuit components. Therefore, die thickness on the order of 0.011 inches are employed, representing the maximum die thickness that can easily fit within a smart card package. Semiconductor die thinner than 0.011 inches are typically not used in smart cards, as such die have traditionally been difficult to handle during the manufacturing process, and the resulting manufacturing expenses are relatively high. Furthermore, conventional wisdom dictates that, as the thickness of a die is decreased, the die become increasingly vulnerable to mechanical failure.
A shortcoming of existing 0.011-inch die is that the die do not provide sufficient immunity to mechanical flexure. When such die are used to fabricate smart cards, breakage and card failure may result if the smart card user bends or flexes the card. Accordingly, flexure is an especially important physical property to consider for smart card applications. In order to improve performance in this area, existing approaches have focused on strengthening the 0.011-inch die through the optimization of specific individual design parameters, such as grinding parameters, dicing parameters, and others. As opposed to integrating these design parameters into a broad-based design solution, typical approaches have adopted a piecemeal approach by considering the effects of only one or two design parameters on flexure resistance. For example, in material systems having high thermal coefficients of expansion, design parameters have been optimized for the purpose of increasing die tolerance to severe thermal transient conditions.
Another shortcoming of existing smart card semiconductor die designs is that little, if any, consideration is given to RF (radio frequency) performance issues. For example, one presently-available smart card requires direct mechanical and electrical contact during use, whereas another type of smart card uses signals in the extremely-low-frequency (ELF) area of the RF spectrum, in the range of 300 to 20,000 Hz, with existing industry-standard UART protocols of 2400, 4800, 9600, and/or 19,200 band. Existing smart cards do not operate at frequencies above the ELF region. Although transponder devices and pagers have been developed for use at higher frequencies, such devices occupy a much larger physical volume than is available within the confines of a smart card. Meanwhile, in relatively recent times, high-speed microprocessors operating at speeds of around 100 Mhz have been developed, and radio frequencies in the 800 and 900-Mhz regions of the frequency spectrum are now enjoying widespread use.
Consider a two-inch lead used in an existing smart card package. This lead provides negligible inductive reactance at 1 Khz, on the order of a fraction of an ohm. That same lead, used at 500 Mhz, provides an inductive reactance of several hundred ohms, which may severely disrupt desired circuit operations at higher frequencies. Moreover, when an existing semiconductor dice having a thickness of 0.011 inches is used to fabricate active semiconductor device, these devices provide electron transit times on the order of several tenths of microseconds, effectively limiting device operation to frequencies less than about 10 Mhz.
Existing field-effect transistors for use in the UHF and microwave regions of the RF spectrum use die thicknesses in the order of 0.00236 inches, so as to provide a relatively short electron transit time. These short electron transit times provide increased high-frequency performance. One technique for fabricating these field-effect transistors is described in U.S. Pat. No. 5,163,728 issued to Miller and entitled, xe2x80x9cTweezer Semiconductor Die Attach Method and Apparatusxe2x80x9d. Unfortunately, the methods and systems described in the Miller patent are only practical when used to construct discrete transistor devices. The use of tweezer-based devices to construct smart cards is impractical because it would be much too labor-intensive, time-consuming, and expensive. What is needed is an improved technique for constructing a smart card that has enhanced RF (radio frequency) properties.
Smart card packages are about 0.030 inches thick, thereby providing a package that is very similar in dimensions to that of a conventional credit card. Note that existing smart card packaging techniques place the semiconductor die near the surface of the card, due to tight packaging and interconnect requirements, and also because the thickness of the die represents a substantial portion of the thickness of the actual smart card package. Therefore, if a user bends a smart card back and forth, the semiconductor die, being situated near the surface of the card, is subjected to relatively high levels of mechanical stress.
RF coupling, as opposed to direct physical contact, is a more advantageous technique for sending and receiving data to and from a smart card, in terms of user convenience and smart card reliability. However, semiconductor die material functions as a lossy dielectric, attenuating RF signals that are incident thereupon, including the signals that are used to couple data to and from the smart card. This attenuation limits the maximum coupling distance between a smart card and a smart card reader, and also restricts the position in which a smart card must be held relative to a smart card reader/writer, in order to successfully read and write data from and to the smart card. The attenuation is substantially proportional to the thickness of the semiconductor die used to fabricate the smart card, inasmuch as the smart card packaging material is a nonconductive plastic encapsulant offering very minimal RF attenuation, and the conductive leads to and from the semiconductor die occupy an inconsequential portion of the smart card package.
Improved smart card semiconductor die are provided that have a thickness of approximately 0.004 to 0.007 inches. These die are positioned at or near the neutral plane (i.e., plane of substantially zero mechanical strain during flexure) of a smart card, thereby providing smart cards having improved resistance to mechanical flexure and/or enhanced performance at RF frequencies.